Difference between two models, objects disappear from view, cameras added?
In your 1st blend, you've checked "lock camera to view" in the "N" panel. This could have caused some strange things on the view and camera position.

You can't see your (flat) sphere because the sphere and camera are at the exact same Z position.

Move your camera up or your sphere down (on Z axis, use G, Z to constraint the movment on Z axis) and you'll be able to see it.